Stem cell differentiation is known to involve changes in RNA expression, but little is known about translational control during differentiation. We comprehensively profiled gene expression during differentiation of embryonic stem cells (ESCs) into embyroid bodies (EBs) by integrating conventional transcriptome analysis with global assessment of ribosome loading. Differentiation was accompanied by an anabolic switch, characterized by global increases in transcript abundance, polysome content, protein synthesis rates and protein content. Furthermore, 78% of expressed transcripts showed increased ribosome loading, thereby enhancing translational efficiency. Elevated protein synthesis was accompanied by enhanced phosphorylation of eIF-4E binding protein, suggesting regulation by the mTOR pathway. Some transcripts were under exclusive translational control, including Activated Transcription Factor 5 (ATF5) a b-zip transcription factor, Deleted in Colon Cancer (DCC) the tumor suppressor and Wnt1, the beta-catenin agonist. Parsimonious translation in ESCs may provide a layer of quality control to prevent inappropriate gene expression in the pluripotent state. Experiment Overall Design: Embryonic stem cells (ESC) maintained in an undifferentiated state and day-5 Embryoid bodies (EB) were selected for RNA was extraction and hybridization on Affymetrix 430_2 mouse expression arrays. For polysome fractionation, twelve fractions collected from the gradients were combined to form four pools. One ml of each pool (Pools: 1-4) was used for RNA isolation, labeling and hybridization for undifferentiated ESCs (UD1, UD2, UD3 and UD4) and EBs (EB1, EB2, EB3 and EB4). In parallel, total RNA was also isolated from unfractionated lysates for transcript abundance analysis. Three biological replicates of ESCs and EB cultures were used for polysome fractionation and total RNA analysis.